Porcelain enamels are the coatings of choice for various metal substrates including cast iron, steel, and aluminum. These enamels in the form of wet slips or dry powders are applied to metallic substrates which are then subject to high temperature firing to provide a smooth porcelain enamel coating. Depending on the specific requirements for certain applications, porcelain enamels can protect metal substrates from corrosion, oxidation, and abrasion. They can also form an aesthetically appealing surface on the metal substrate to provide desirable color, gloss, and unique surface texture.
Traditionally, porcelain enamels have been divided into ground coats and cover coats. A ground coat serves as an intermediate layer, providing a bond between the base metal substrate and the cover coat. A cover coat is designed to provide color and surface characteristics that are combined with other required physical properties such as resistance to corrosion, abrasion, and thermal shock.
It is highly desirable to combine the functionalities of both ground coat and cover coat into one single coat. In particular, black color dual purpose porcelain enamel glasses are in demand for enameling non-pickled steels. These dual purpose enamel glasses which are typically applied by the well known electrostatic powder spraying technique, simplify the processing requirements, minimize waste generation, and reduce production cost.
To produce black color dual purpose electrostatic powders, cobalt-bearing glasses are normally used. Cobalt oxide (CoO), as well as nickel oxide (NiO), is considered as a necessary promoter of adherence between glass and steel. Cobalt oxide is also a colorant for black glasses. The art has long recognized that iron oxide (Fe.sub.2 O.sub.3), among other transition metal oxides such as copper oxide (CuO) and manganese oxide (MnO.sub.2) may be used in enamel compositions to impart various shades of color to the enamel. Normally these transition metal oxides are used at levels below 3 wt %. Higher concentration would lead to crystallization upon the firing of the enamel.
Cobalt-free enamel glasses were developed in response to the critical shortage of cobalt raw materials in the world market in the 1970's. Typically, such cobalt-free glass compositions were formulated by substituting all the CoO in a cobalt-bearing composition with NiO. In these compositions, the ratio of alkali (Li.sub.2 O:Na.sub.2 O:K.sub.2 O) is also adjusted to attain proper viscosity of the enamel at the firing temperature.
Current commercial cobalt-free enamels are cheaper substitutes for cobalt-bearing glasses in general purpose ground coats whose primary function is to provide good adherence to the metal substrate. Their applications are limited in many cases because of the lack of color stability, thermal durability and acid resistance. To expand the applicability of cobalt-free enamel glasses, it is necessary to make them darker, to impart greater color and thermal stability and to improve their resistance to acid.
The effect of iron oxide on the darkening of enamel color has been studied: as the iron content increases, the reflectance of the glass continuously decreases, reaching a value comparable to that which is obtainable with typical cobalt-bearing compositions. Iron oxide, which is often considered as an undesirable discoloring impurity component in glasses, can function both as a glass network former and a network modifier. It was also reported by Kruchinin et al. ("Effect of iron oxides on the structure and properties of sodium borosilicate enamels", Glass & Ceramics, pp 364-367 (Translated from Steklo i Keramika, No. 9, pp 23-24, September, 1990, Plenum, New York) that iron oxide has a positive effect on the properties of multi-component sodium borosilicate enamel coatings in reducing the firing temperature, and increasing the impact strength, wear resistance, and acid resistance.
The use of iron oxide in enamels has been disclosed in several Russian patents. The compositions thus disclosed are unlike the present invention and are suitable only for wet systems where the desirable acid and water resistance and thermal durability are imparted by adding clays and refractory mill additions. In electrostatic powder application, however, such desirable properties have to be built-in within the enamels since mill additives cannot be used for reasons of encapsulation.
Russian Patent 1,014,807 disclosed cobalt-bearing flits. Russian Patent 1,265,160 disclosed frits lacking acid resistance and thermal durability. The composition contains 35-49% SiO.sub.2, 14-18% B.sub.2 O.sub.3, 2-5% Al.sub.2 O.sub.3, 1.5-5% CaF.sub.2, 2-4% K.sub.2 O, 15.6-20% Na.sub.2 O, 0.4-0.6% CuO, 9-12% Fe.sub.2 O.sub.3, 1.4-1.6% NiO, and 0.9-1.1% MnO. Russian Patent 1,470,685 disclosed frits containing 0.1-2% Cr.sub.2 O.sub.3 and 0.1-1% Mo.sub.2 O.sub.5. The frit composition also contains 29-35% SiC.sub.2, 12-16% B.sub.2 O.sub.3, 8-12% Al.sub.2 O.sub.3, 20-24% Na.sub.2 O, 1.7-6% Fe.sub.2 O.sub.3, 1-3% NiO, 3-9% CaF.sub.2, and 0.1-1% CuO. Russian Patent 1,609,756 disclosed frits containing 2.3-4.1% Cr.sub.2 O.sub.3, 6.8-12% ZnO, and 0.1-1% MoO.sub.3. The composition also contains 28-36% SiO.sub.2, 1.5-4% TiO.sub.2, 14-18% B.sub.2 O.sub.3, 1-4% Al.sub.2 O.sub.3, 4-8.55 P.sub.2 O.sub.5, 15-20% Na.sub.2 O, 2.5-5% Fe.sub.2 O.sub.3, 1-3% NiO, 3.-9% CaF.sub.2, and 0.1-1% CuO.